1. Field of the Invention
This invention relates to fuel ignition systems of the intermittent pilot type, and more particularly to a control circuit for use in such systems which provides fail-safe control of fuel valves of the system, and in which pilot flame sensing is effected using the spark electrodes.
2. Description of the Prior Art
In fuel ignition and control systems of the pilot ignition type, a pilot valve is operated to supply fuel to a pilot outlet for ignition by sparks provided by a spark generating circuit to establish a pilot flame. A pilot flame sensing circuit detects the pilot flame and effects the energization of a main valve to supply fuel to a main burner for ignition by the pilot flame. In such arrangement, the main valve is operated only when the presence of the pilot flame is proven.
Typically, the flame sensing circuit includes a separate flame sensing probe which is mounted in the proximity of the pilot outlet. When the pilot fuel is ignited, the pilot flame impinges on the sensing probe, permitting current to flow through the flame sensing network, generating a signal, commonly referred to as a flame signal. The flame signal is applied to a main valve control circuit to effect energization of the main valve.
While the use of a flame probe and its associated flame sensing network provides a convenient way of detecting the presence of a pilot flame, in some gas furnace installations, there may be insufficient space to permit mounting the separate flame sensing probe near the pilot outlet or at least close enough to the outlet to permit the flame to impinge on the probe. Also, the need for the separate flame sensing probe and the probe connecting wire adds cost to the system.
It would be desirable to have a fuel ignition and control circuit which provides fail-safe operation of the fuel valves of the system which does not require a separate flame sensing probe for detecting the pilot flame.